A cost-effective method of reducing loose grain damage during the bating process

Introduction
Enzymatic bating is an important part of the leather making process. Bating enzymes help remove unwanted skin components from the hide structure, as well as any scud present on the grain layer. Good bating leads to clean, soft leather. Under bating results in poor removal of unwanted skin components, and a firmer crust leather. Bating for too long can result in a sueded effect on the grain layer as well as loose grain. Tanners therefore need to exercise caution during the bating process, and the problem is that judging the endpoint of bating is a subjective process.

Trypsin is the most common bating enzyme used. It has high activity in a weak alkaline pH (pH 8 – 9). It has a high molecular weight, and therefore does not penetrate well into the hide. When added to the float of a drum during the bating process, it takes a long time for it to penetrate into the hide. However, because it is highly active at the float pH of bating, it can hydrolyse the grain layer long before it penetrates through the cross-section of the hide. This can lead to grain damage and loose grain.

Previous experiments have shown that using the trypsin inhibitor Kunitz trypsin inhibitor (KTI) at the beginning of the bating process in conjunction with the bating enzyme, was able to reduce grain damage and loose grain during the bating process. KTI is, however, expensive. Soybeans naturally contain KTI and are a low-cost crop. If soybeans are ground into flour, this soybean flour (SF) could be used as a source of KTI to inhibit the trypsin bating enzyme. This research was carried out to determine if SF could be added to the float during the bating process and have beneficial effects on the hides by protecting them from enzymatic damage to the grain layer during the bating process.

Experimental
After first optimizing the amount of SF to inhibit trypsin activity, two groups of delimed hides were bated with 0.1% trypsin in a float of 100% water at 32°C for 2 hours. After the two hours of conventional bating, SF (5.16%) was added to one of the drums to inhibit the action of the bating enzyme on the grain surface of the hides. The other drum did not receive any SF and was the control. The hides in both drums were then bated for a further 6 hours at 32°C. On completion of the bating process, the hides were then pickled, tanned and post-tanned through to crust leather.

The grain surfaces of the crust leathers were observed using field emission scanning electron microscopy. The physical properties such as softness, tensile strength and tear strength of the crust leathers were measured after conditioning at 20°C and 65% relative humidity for 24 hours.

Results
The effect of the SF of the grain quality of the crust leather was evaluated by comparing the surface of the leather that had been bated with the trypsin enzyme only (control hides), compared to the leathers bated for 2 hours, after which the SF was added to the float for the remainder of the bating time.

The SEM images of the grain surfaces (images not shown) showed that the hair follicles were more intact in the hides treated with the SF, compared to the control group that had been bated with trypsin enzyme only. However, there was no visible difference in the fibre structure between the middle layers of the hides treated with SF compared to the control hides. These images showed that the SF protected the grain layer of the hides during bating but had no effect on the bating action of the trypsin enzymes in the middle layer of the hides.

The results in Table 1 show that as the length of bating time increases, this increases the softness of the crust leather. The softness results were similar for both the crust leather bated with trypsin only compared with the leather bated with trypsin and the SF. This proved that the SF did not appear to weaken the bating activity in the middle layer of the hide.

The tensile and tear strength results shown in Table 1 show that the tensile and tear strengths of the crust leather decreased with increasing bating time. However, it is also clear that this decrease was less in the crust leather that was treated with SF. This shows that the SF reduced damage to the collagen in the grain layer of the hides, which is important to the strength of the leather.

Table 1. Physical and softness properties of the crust leather

Conclusion
The researchers found that soybean flour could be used to inhibit the activity of trypsin enzyme after a certain period of bating time. This inhibition took place in the grain layer of the hide and did not affect the action of the bating enzyme in the middle layer of the hide. As a result, the grain layer of the hide was not negatively affected by the bating enzyme despite bating up to 8 hours long, but bating continued to take place in the middle section of the hide. This research provides an environment-friendly, low-cost method of producing high quality leathers as soybean flour is readily available and has a low cost. After a certain period of bating time, it could easily be added to the float in the drum, and would protect the hides in the drum from enzymatic damage to the grain layer.

Source: This article is a summary of the paper “Preventing Enzymatic Damage to Hides by Timely Inhibition of Trypsin Activity with Soybean Flour during Bating Process.” Journal of the American Leather Chemists Association (JALCA) Vol 118, 245 - 252 (2023).